It has been shown in the past that two-dimensional radar dimensional radar images of ship and air targets can be obtained by using a process referred to as inverse synthetic aperture radar (ISAR). In this two-dimensional technique the target of interest is observed with a radar capable of resolving the target in both slant-range and cross-range. Slant-range is extracted by processing wideband stepped-frequency or pulse-compression waveforms. Cross-range is extracted by processing the fine-structure differential Doppler produced by target aspect motion. By employing a stepped-frequency waveform applicable to a wide range of radars, the slant-range resolution is limited only by the radar's electronically tunable bandwidth.
From the standpoint of image interpretation, however, the following inherent shortcomings remain in the ISAR technique:
1. The cross-range dimension scale is a direct function of the target's aspect rotation rate. The result is distorted images unless the rotation rate can be determined from auxiliary target track filed data.
2. The image plane is unknown since the radar cannot determine the direction of the target's rotation vector producing the differential Doppler.
3. The required image frame time to produce a given cross-range resolution is dependent on a target's aspect rotation rate relative to the radar. Therefore, a long-range, nonmaneuvering air target may require tens of seconds to image.
These three problems with the ISAR technique result from its inherent dependence upon target changing its aspect to the radar. The magnitude and direction of the target's aspect rotation vector are not determined in the ISAR approach except by auxiliary data, and the magnitude may be too small.